Does maternal condition affect the sex ratio of offspring in humpback whales

Abstract Abstract. The hypothesis that maternal condition can affect the sex ratio of offspring was investigated by examining the sex of calves born to mature female humpback whales, Megaptera novoeangliae , in the southern Gulf of Maine. Maternal condition was inferred from calving interval: females calving at intervals of 3 or more years were judged to be in 'superior' condition, while those calving at the modal 2-year interval were considered to be in 'average' condition, and those at an interval of 1 year in 'poor' condition. The sex ratio of calves horn to females in superior condition deviated significantly from the expected 1:1 (χ 2 = 8·05, N = 21, df = 1, P 2 = 0·09, N = 42, df = 1, P >0·7). Interpretation of these results according to the Trivers & Willard (1973, Science , 179, 90-92) hypothesis would suggest that mature female humpbacks in superior body condition bias their reproduction towards the sex (males) with the greater variance in reproductive success, although rejection of the central assumption of this study (that a longer calving interval results in superior maternal condition) would render such an interpretation invalid.